The invention relates generally to heart-monitoring systems and more particularly to the improved suppression of pacemaker artifacts appearing in ECG signals.
In the art of heart-monitoring, thoracic electrical potentials of the patient are sensed to provide an input signal to amplifying means which amplify and process the signal for application to various utilization devices including recorders, CRT displays, heart-rate indicating means, and the like. In the particular instance of heart-rate monitoring, the heart-rate indicator is normally responsive only to sensed signals which correspond in frequency and amplitude substantially with the QRS complex or the R wave of a natural heart beat appearing in the ECG. However, in the event the patient is receiving artificial stimulation as by a heart pacemaker, the sensed electrical signals resulting directly from the pacer may additionally introduce artifact signals having amplitudes and/or frequencies which may be inaccurately identified as a naturally occurring heart rhythm and, in the worst case, may provide an indication of continuing heart activity when, in fact, heart activity has ceased and the patient is technically dead. This erroneous determination that a pacer signal artifact constitutes a normal QRS complex may be made not only by heart-rate indicating means, but also by other analytical means including human observers. For this reason, it is important to suppress pacer signal artifacts which might otherwise be misinterpreted as a functioning of the heart.
The pacer signal artifact normally comprises a stimulation pulse portion, or "spike", representative of the discharge of capacitively-stored electrical energy into the patient's heart and generally also a recharge waveform portion or "tail" attending the recharge of the pacer's energy-storing capacitor.
The prior art has provided various circuit means for suppressing the "spike" portion of the pacer signal artifact, usually by preventing transmission of the sensed signal to the utilization circuitry for the duration of the pacer discharge pulse. Such circuits have, however, generally been relatively complex and/or subject to drift and/or muscle noise or artifact.
Recently, means have been provided for also suppressing the pacer recharge waveform or "tail", as described in greater detail in U.S. application Ser. No. 760,487 for PACEMAKER ARTIFACT SUPPRESSION IN CORONARY MONITORING BY Marchese et al., filed Jan. 19, 1977, now U.S. Pat. No. 4,105,023 and which is incorporated herein by reference to the extent consistent herewith. Briefly, the "tail" suppression circuitry of the aforementioned application utilizes a differentiating circuit to recognize the occurrence of a pacer pulse and to control the timing of an interval associated with pacer recognition circuitry such that immediately following the end of the "spike" portion of a recognized pacer signal artifact, the pacer "tail" portion is sampled and the amplitude of the sample is utilized for generating a "tail" suppression signal of opposite polarity to the "tail" portion of the pacer signal artifact. The "tail" suppression signal is arithmetically added with the ECG signal containing the pacer signal artifact such that the pacer "tail" portion thereof is reduced in amplitude and substantially cancelled, thereby avoiding the possibility of its actuating rate-indicating circuitry or the like.
Regarding the prior technique and circuitry for suppressing the "tail" portion of the pacer signal artifact as disclosed in the aforementioned U.S. patent application, it will be appreciated that generation of the "tail" suppression signal required an amplitude measurement to be made of the "tail" immediately following termination of the "spike" portion of the pacer signal artifact. In order to accomplish this, it was first necessary to determine that a high frequency signal in the sensed ECG was indeed a pacer artifact signal and subsequently to accurately identify the brief interval during which the amplitude of the "tail" is to be sampled. The circuitry employed for such technique tends to be complex.
Accordingly, it is a principal object of the present invention to provide an improved means for the suppression of pacer signal artifact in a heart-monitoring system. Included in this object is the provision of means for preventing false actuation of heart-rate indicating means by any portion of a heart pacer signal artifact.
It is a further object of the present invention to provide improved means for suppressing the recharge waveform portion of a pacer signal artifact appearing in a sensed ECG signal. Included in this object is the provision of relatively simple, reliable and low-cost means for the suppression of the recharge waveform portion of a signal artifact.
It is a still further object of the present invention to provide improved pacer signal artifact suppression means with minimal response to muscle artifact.
It is an even further object of the present invention to provide improved means for the suppression of the discharge pulse portion of a pacer signal artifact.
These and other objects will be in part obvious and in part pointed out in greater detail hereinafter.